upipe_sws: fix incorrect pointer init
Add upipe_sws_thumbs
Fix upipe-sws doc
Add upipe_avcdec/upipe_avcenc

- upipe_avcdec is a renamed/cleaner upipe_avcdv
- upipe_avcenc is an avcodec encoder embryo
upipe_av_pixfmt.h: multiple PixelFormat

Support multiple compatible PixelFormat per picture format
(PIX_FMT_YUV420P/PIX_FMT_YUVJ420P for instance)
transcode_x264 -> transcode_video

Add avcodec encoding support to transcode example, rename it.
Remove upipe_avcdv (replaced by upipe_avcdec)
upipe_avcenc: fix memleak